The present invention relates to a node (router) that is capable of connecting communication networks such as for example Ethernets and that is capable of transferring packets at the frame level defined in the datalink layer, and to a method of frame transfer.
A router device is used for connecting LANs (Local Area Networks) and transfers datagrams from one LAN to another LAN. In addition to the communication information data to be transferred, the datagram includes the network-layer addresses of its source and destination (i.e. source IP address and destination IP address in a case of IP (Internet Protocol)). The router determines a next-hop node (router or host) and an output interface of the datagram based on the destination address.
In a conventional router, when datalink frames (for example, Ethernet frames) defined in the datalink layer (for example, MAC (Media Access Control) layer) are received from one LAN, these frames are first subjected to the datalink-layer processing, then converted into a packet for being subjected to the network-layer processing.
The output interface and the network-layer address of the next-hop node of this packet are then determined from the network-layer destination address contained in this packet, so that this packet can then be handed over to the corresponding output interface. At this output interface, the datalink-layer address is determined from the network-layer address of the next-hop node and then the packet is converted into datalink frames to be output to another LAN.
The router that transfers communication information (datalink frames) by the procedure described above, after converting received datalink frames into a network-layer packet, determines a node to which this packet is next to be sent (next-hop router or destination host) and an output interface corresponding to the next-hop node, by referring to the content of this packet such as the network-layer address of the final destination (and source address) and by using a network-layer routing table held in the router, and then converts the packet into datalink frames once more to output the frames to the next-hop node.
In the conventional router, the excessive amount of processing within the router involved in the conversion of received datalink frames into a packet, lookup of the network-layer routing table, and conversion of the packet into datalink frames resulted in a large processing time being required for the frame transfer and in a large processing load required within the router.